An existing process for zinc production involves roasting of a zinc containing ore, followed by acid sulphate leaching and then electrolytic recovery of zinc from the leachate. The roasting process produces sulphur-based pollutant gases which must be removed from the roasting furnace exhaust. In addition, if the ore contains high levels of impurities this can adversely affect the electrolysis and purity of zinc produced. For example, many zinc ores contain significant quantities of manganese. The existing process is constrained in its ability to treat zinc mineral concentrates containing significant manganese, as the manganese leaches into the leachate along with the zinc and cannot be effectively removed. Manganese causes problems in the zinc electrolytic recovery step by depositing on the anodes as MnO2.
In Australian Patent 669906, the present applicants developed a multi-stage leaching process followed by electrolysis for the recovery of copper. Conversely, U.S. Pat. No. 4,292,147 discloses a process for the recovery of zinc using chloride leaching followed by electrolysis.
The present inventors have also discovered that, when solution, manganese does not deposit on the anode during electrolytic metal recovery from the solution, especially with solutions derived from the leaching of zinc and/or lead ores. This allows for the manganese to be removed from the solution by other means.
Furthermore, in hydrometallurgical processes that have a high solution halide concentration (eg. from 200 to 300 grams per litre NaCl and 10 to 50 grams per litre NaBr), it is desirable to remove deleterious impurities prior to the electrowinning of various metals such as zinc or copper in the process. Many impurities can be removed from the electrolyte solutions of these processes by a stepwise pH adjustment and precipitation (eg. up to around pH 6), however, both silver and mercury are not so readily removed.
U.S. Pat. No. 4,124,379 discloses the removal of silver from a cuprous chloride electrolyte, whereby copper metal is added to the electrolyte to reduce cupric ions to cuprous ions followed by contact with an amalgam that exchanges a metal for the silver. The amalgam is formed from mercury metal with one of copper, zinc or iron, preferably copper shot coated or associated with mercury metal. The amalgam requires separate production, requiring a physical association with the mercury and copper, and this is cumbersome and complex, adding to the cost of the process. In addition, the amalgam must then be added to the process and contacted with the silver-bearing electrolyte, increasing the complexity and cost of running the process.